The treatment of hides and skins to form leather involves a number of interdependent chemical and mechanical operations. These operations may be divided into a sequence of wet end steps followed by a sequence of dry steps. A typical leather making process involves the following sequence of wet end steps: trimming and sorting, soaking, fleshing, unhairing, baiting, pickling, tanning, wringing, splitting and shaving, retanning, coloring, fatliquoring and setting out. These wet end steps are followed by a sequence of dry steps such as drying, conditioning, staking, buffing, finishing, plating, measuring and grading. A description of each of these operations is provided in Leather Facts, New England Tanners (1972).
The present invention is involved with the wet end operations which take place after primary tanning; namely retanning and fatliquoring. The object of primary tanning is to convert the hide or skin to a stable non-spoilable material. This is accomplished by converting raw collagen fibers in the hide or skin into a stable product which is non-putrescible or in other words will not rot. In addition, tanning improves a number of properties of the hide or skin such as for example, dimensional stability, abrasion resistance, resistance to chemicals and heat, improved flexibility, and the ability to endure repeated cycles of wetting and drying. The principal method used to tan hides and skins is known as "chrome tanning". This employs a basic chromium sulfate, often referred to simply as "chrome", which is prepared by the reaction of a chromium salt, like sodium bichromate, with a sugar-like substance and sulfuric acid. The chrome penetrates into the skin producing a bluish-green color. The color change is used to assess the extent of penetration or degree of tanning. In addition, the shrinkage temperature is used to measure the rate and degree of tanning. Untanned leather will shrink significantly when subjected to hot water, as for example 140.degree. F. water, while properly chrome tanned leather can withstand higher temperatures, such as for example 212.degree. F. water, without shrinking. For a description of chrome tanning see U.S. Pat. No. 4,327,997. Hides and skins may also be tanned using vegetable extracts for example extracts from trees and shrubs such as quebracho, wattle, sumac, hemlock, oak and spruce.
After tanning, the leather is retanned, colored and fatliquored. This three step operation is often considered together as one step since all three operations may be carried out sequentially in one drum Chrome-tanned stock, also referred to as "blue stock", retains much of the uneven fiber structure pattern in the skin on the animal. Some areas of the skin possess a dense structure while other portions are loosely fibered and some portions may be undesirably thin and papery. Since the tanner desires to produce a uniform piece of leather, a second tanning step, known as "retanning" is employed to improve both aesthetic and physical properties. These properties include, for example, improvements to the fullness of the leather, the tightness and smoothness of the grain, the break, the levelness and intensity of the dye shade, better uniformity in temper or flexibility, better wettability and additional stability against water and perspiration. Retanning can be accomplished using a variety of naturally derived materials including extracts from vegetables or plants, and synthetic tanning agents known as "syntans", or combinations thereof. Historically, extracts from trees and shrubs like quebracho, wattle, sumac, hemlock, oak and spruce were used as retanning agents. Over the past 50 years many man-made syntans were developed and these are used extensively today, especially for manufacturing soft-leather and making white or pastel color leathers. Retanning is typically conducted at temperatures of from about 80.degree. F. to about 120.degree. F. using from about 3 to about 20 weight percent of retanning agent on the wet weight of the tanned leather. In some instances the hide may be chrome retanned before the regular retanning step to fully tan any previously untanned portions and to level out the chrome especially in the grain for more uniform dyeing. Retanning typically takes on the order of 1 to 2 hours, while the entire retanning, coloring and fatliquoring sequence usually takes from about 4 to 6 hours. After retanning the hide is colored using either a surface type dye or a penetrating dye. In general, acidic dyes penetrate through the hide while basic dyes are used to color only the surface.
After retanning and coloring the hide is then subjected to the fatliquoring step. Fatliquoring imparts the desired properties of stength and temper to the leather. The fatliquor lubricates the leather fibers so that after drying the fibers are capable of sliding over one another. In addition to regulating the pliability of the leather, fatliquoring contributes greatly to the tensile and tearing strength of the leather. Fatliquoring also affects the tightness of the break or in other words the crease pattern formed when the grain surface is bent inward; the object being to produce a leather which leaves no or few fine wrinkles when it is bent.
The basic ingredients used in fatliquoring are water insoluble oils and fatty substances such as raw oils and sulfated and sulfited oils. Typically the weight percent of fatliquor oil on weight of leather ranges from 3 to 10 percent. The manner in which the oil is distributed throughout the leather affects the character of the leather and subsequent finishing operations. To obtain a uniform oil coating over a large surface of leather fibers it is necessary to dilute the oil with an organic solvent or preferably to disperse the oil in an aqueous system using emulsifiers. See Leather Technician's Handbook, J. H. Sharphouse, Leather Producers' Association (1971) chapters 21 and 24.
While techniques directed to controlling the degree to which the emulsion penetrates the leather before breaking and depositing as oil on the fibers have been employed to make leathers softer and more flexible, long term water resistance or waterproofness has not been successfully accomplished using conventional fatliquors alone.